GameMode has a lot of functionality ready for you. It can give you a head start when dealing with multiplayer games where players will be connecting together to play in the same match (Like an FPS with rounds). In your case, GameMode is still probably right for you. If you derive from GameModeBase then you need to write the functionality for handling match states yourself. You’ll have to do some panning through the GameMode class to learn how to use it, but it’s pretty straight forward.
For example: your coop game would need to wait for both players to join, then start the match after both are ready. GameMode has these states for you and it works along side GameState as well. Additionally, it has functionality for hanging on to players if they happen to disconnect or something. It will store the player state and you can use that to give it back when they reconnect.